The present invention relates to a pump for high temperature liquid, particularly a boiler feed pump for pumping high temperature water to a boiler, which has a rotary seal having cooling means to keep the seal temperature within its recommended range.
Boiler feed pumps for power stations typically pump water at elevated temperatures (150.degree. centigrade and greater) and at elevated pressures. Such pumps are usually driven by turbines or electric motors. The pump has a number of impellers on a driven shaft which progressively pressurize the feed water up to pressures of typically 150 to 300 bar. The shaft is provided with rotary mechanical seals at either end, sealing the shaft into the pump housing. These seals, however, require cooling in order to operate satisfactorily and have a reasonable lifetime.
In the past, the seals have been cooled by means of a cooling jacket located inboard of the seals through which coolant is passed from an external source. Additionally the seal is commonly cooled by means of a closed loop cooling circuit. By this means, boiler pump water within the space occupied by the mechanical seal is pumped through a cooler by means of a pumping ring mounted on the shaft. Thus the temperature of the seal is kept lower than the rest of the pump whilst the pump is running.
When the pump is at rest, there is still a requirement to maintain the mechanical seals at a lower temperature than the rest of the pump. This is effected by continuing to circulate coolant through the aforesaid cooling jacket augmented by circulation of boiled feed pump water within the closed cooling loop by means of natural convection.
The temperature differentials arising, during this hot standby condition, from the need to cool the mechanical seal result in vigorous convection currents being set up in the annular space between the space and the cooling jacket inboard of the seal. This condition leads to local thermal distortion of the stationary shaft, which tends to become bowed.
Thermal bowing of the shaft has certain undesirable consequences. Firstly, there may be premature wear of internal clearances when a pump on on hot standby is started. Secondly, on a turbine driven pump being subjected to low speed barring operations, the friction torque, or even seizure, that develops when barring is discontinued for any reason can be sufficient to prevent reinstatement of barring. At the least, it will take several hours for the pump and its contents to cool sufficiently to permit barring reinstatement. Thirdly, the bowed shaft gives rise to mechanical imbalance and vibration on start up which will persist until the shaft temperature becomes uniform. Fourthly, the equalibrating forces between the thermally bowed shaft and constraining internal and journal bearing clearances give rise to orbital motion of the shaft journals within the bearings. This condition can result in the pump being tripped out if shaft displacement safety sensors are installed.
For these reasons, it is desirable to reduce or eliminate the thermally induced bowing of the shaft during hot standby.
Several attempts have been made to do this in the past. On turbine driven pumps, a barring mechanism is provided which continually rotates the shaft at a slow speed during hot standby. This is a requirement of the turbine itself. However, there are significant control problems if barring gear is fitted to electric motor driven pumps where motor start up is automatic. Then, special provision must be made to disengage the barring gear as part of the start up sequence and this involves the additional risk of wrecked barring gear should disengagement fail to take place.
Another approach to this problem has been to inject hot boiler feed water from other operational pumps directly into the annular space between the shaft and housing of the pump on hot standby. This inhibits the convection currents which lead to bowing of the shaft. However, the effect of this is to maintain the seal area at an undesirably high temperature resulting in premature deterioration. Cooler water from another source could be injected to overcome this problem but the resulting additional equipment and complexity involved is undesirable.